Spherical powders

ABSTRACT

The instant disclosure teaches an improved process for obtaining dense, spherical, metal particles characterized by coating the starting material with a weld-preventing material and heating the coated particles in a protective atmosphere.

O Umted States Patent [151 3,652,259 Knopp [4 1 *Mar. 28, 1972 [54] SPHERICAL POWDERS [72] Inventor: Walter V. Knopp, Wyc koff, NJ. [56] ndenm cited 73 Assignee: 01in Mathhson Chemical Corp. I UNITED STATES PATENTS y 1 Notice: The p i f the term f i patent MOnSOn B M 25 1986 has been 3,214,264 10/1965 Bogdandy. ....75/0.5 B v 3,434,831 3/1969 Knopp ..1s/212 3,214,262 10/1965 Bogdandy et al.... ....75/0.5 BA [22] Filed: May 14, 1968 3,036,938 5/1962 Hudson ..75/0.5 B [2]] Appl 7283 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Robert H. Bachman and Richard s. Strickler [63] Continuation-impart of Ser. No. 577,840, Sept. 8,

1966, Pat. No. 3,434,831. [57] ABSTRACT The instant disclosure teaches an improved process for obtain- [52] 0.5. CI. ..75/0.5 B, 75/0.5 BA, 75/265633, in dense spherical meml Particles characterized by coating the starting material with a weld-preventing material and heat- [5 l Int. Cl. B221 9/00, B29C 23/00 g the oated particles in a protective atmosphere. [58] Field 01 Search ..75/.5 B, .5 BA, .5 BB, .5 BC; 264/15 5 Claims, No Drawings SPHERICAL POWDERS This case is a continuation-in-part of U.S. patent application, Ser. No. 577,840, by Walter V. Knopp, filed Sept. 8, 1966, now U.S. Pat. No. 3,434,831.

Numerous processes have been used for obtaining rounded or spherical metal powders or metal shot. The disadvantages of these processes are many fold. In particular, it is difficult to obtain a closely controlled particle size. Conventional processes are characterized by a wide distribution of particle sizes.

Furthermore, conventional processes are frequently expensive and frequently do not obtain spherical or rounded particles. During atomizing, particles produced by conventional processes are often hollow or have crevices or will be elongated or tear-drop in shape.

In addition to the foregoing there are numerous other disadvantages of conventional processes, which disadvantages will appear in part from the ensuing specification.

For example, a typical art process is the production of shot where the metal flows through an orifice in a thin stream which is then dispersed with high velocity jets of air, steam, water or heated gases.

Alternatively, shot may be produced by pouring molten metal on a rotating surface and then cooling with hot water.

The above methods do not produce a closely controlled spherical size. The yield of round particles is often poor and the yields of desired size will vary.

An alternative method is to cut wire of a predetermined diameter into small pieces. These particles are then partially rounded in a disc mill. This method does insure close particle size; however, the particles are not truly spherical and the material is expensive.

Accordingly, it is a principal object of the present invention to provide a process for the fabrication of rounded or spherical metal powders.

It is a further object of the present invention to provide a process as aforesaid which can obtain a high yield of particles of a predetermined size.

It is a further object of the present invention to provide a process as aforesaid which is inexpensive and readily adaptable to a commercial operation.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention, it has now been found that the foregoing objects may be readily obtained. The process of the present invention comprises:

A. providing metallic particles, preferably copper or copper base alloys, having a particle size larger than 100 mesh,

B. coating said particles with a weld-preventing material, and

C. heating said coated masses in a protective atmosphere above the melting point of said metal to form a plurality of coherent, dense, metal spheres.

In accordance with the present invention, it has now been found that the foregoing process readily obtains a high yield of rounded or spherical particles of predetermined size. The foregoing process is characterized by its relative simplicity and by being inexpensive. Furthermore, the particles obtained in accordance with the present invention are solid and rounded, are obtained in the desired particle size in a high yield, and are generally not hollow and generally do not have crevices.

In general, the process of the present invention makes particles of closely controlled size in very high yields. The process of the present invention makes spherical particles of materials and alloys which are not normally made. The spherical particles may contain metallic additives which do not alloy and also non-metallic additives as well.

In addition, the particles prepared in accordance with the process of the present invention will have a very low oxygen content since melting is done under a protective atmosphere. Thus, for example, copper shot made in this way can be considered as OFHC copper.

The convenient obtaining of the foregoing advantages of the present invention will be readily apparent from a consideration of the ensuing specification.

The present invention may be readily employed with virtually any metallic material. The metallic particles utilized in the process of the present invention should have a particle size larger than mesh. The process of the present invention encompasses numerous modifications which provide a variety of methods for obtaining the starting material of the present invention. These modifications greatly enlarge the commercial applicability of the process of the present invention and represent important facets of the process of the present invention. These will be discussed in greater detail herein below.

The pure metal or alloys may be used. In addition, mixtures of metals may be also conveniently employed or their reducible oxides. For example, it is preferred to use copper or copper base alloys. Others which may be used include aluminum or aluminum alloys, ferrous materials, noble metals, precious metals, titanium, magnesium, zinc, nickel, beryllium, solders, and so forth.

The metallic particles are then coated with any desired weld-preventing material. The particular weld-preventing material is naturally dependent upon the type of metal particles employed. For example, one may readily employ talc, lime, graphite, alumina, titanium dioxide, zirconium oxide, flour, non-reducible oxides, and mixtures thereof.

It is preferred to coat the metallic particles with a binder material prior to the application of the weld-preventing material in order to insure that the weld-preventing material will adhere to the outside of the metallic particles. The binder material may be any material which will readily adhere to the outer surface of the metallic particles and to which the weldpreventing material will readily adhere. In addition, the binder should volatilize above- F. and should volatilize at least 200 F. below the melting point of the metal being used. The particular binder which is employed is not especially critical. For example, one may readily employ brazing fluxes compatible with the base material, waxes, e.g., polyethylene glycol, metal stearates, stearic acid, organic binders, gums, etc. In general, the particular choice of the binder will be influenced by the metal particles being employed. The binder material should volatilize within the foregoing temperature range.

After the metalparticles are coated with weld-preventing material, the coated masses are heated in a protective atmosphere to form a plurality of solid, dense, spherical, coherent, metal spheres. It is preferred to heat the masses in two stages, with the first stage being to a temperature above the volatilized temperature of the binder in order to volatilize the binder, and the second stage being above the melting point of the metal particles in order to form the fully dense spherical particle of the present invention. Naturally, in the first heating step, one should heat above the volatilization-temperature of the binder, but below the melting point of the metal and in the second heating step, one should raise the temperature to above the melting point of the metal particle.

The protective atmosphere which is employed may be neutral on the reducing side. One may use a lean gas or a strongly reducing gas. For example, one may employ hydrogen, carbon monoxide, dissociated ammonia, nitrogen, argon, helium and so forth. Alternatively, a vacuum or partial vacuum may be used as a protective atmosphere.

After the heating step, there is obtained a plurality of distinct, spherical, dense, solid, metal particles predominantly in the particle size range corresponding to the size range of the starting material or slightly smaller. In view of the high yields obtained in accordance with the present invention, it is not normally necessary to rework out-of-size materials; however, if desired, the out-of-size materials may be reworked in the process of the present invention in accordance with the modifications of the present invention which will be disclosed hereinafter.

After the heating step, the particles are cooled and the weld-preventing material removed. Frequently, it is only necessary to screen the particles in order to remove the weldpreventing material. If desired, one may wash and etch the materials.

As indicated hereinabove, the process of the present invention contemplates a variety of ways to obtain the starting material, i.e., a variety of ways for providing the starting metallic particles having a particle size larger than 100 mesh. Some of these methods will be discussed hereinbelow.

Optionally, either a fine metallic powder or a coarse metallic powder may be sintered together to form a cake. The sintering process may also be conveniently utilized to reduce the oxygen content of the metal powder. For example, either copper oxide or a fine copper powder may be furnaced to reduce the oxygen content and to sinter them together to form a coarser particle or cake and to increase the density.

The sintered cake may then be mechanically broken up to produce a material having a particle size larger than 100 mesh. The material may then be processed in accordance with the present invention.

Alternatively, a fine metallic powder can be agglomerated, but not necessarily sintered, using a plastic material which will readily volatilize during subsequent processing. The agglomerated cake would then be broken up to desired size in the manner described above with respect to the sintered cake.

An alternate modification in the process of the present invention utilizes material obtained by disintegrating a stream of molten metal by the use of a fluid. For example, a molten stream of copper can be disintegrated by impingement with water or air. In the so doing, the shot material obtained will contain some oxygen; therefore, when the material is heated up in an atmosphere with hydrogen, the shot will crack because of the pressure of the water vapor formed, i.e., hydrogen embrittlement. If one were to attempt to braze this material, the attempt would be unsuccessful, since the brazing material will flow into the crevices. However, in accordance with the process of the present invention, the shot material may be corrected by treating it in accordance with the process of the present invention. The shot which is treated thereby will contain no cracks and will contain substantially no oxygen. Hence, during brazing, the brazing material will remain on the surface.

The present invention will be more readily apparent from a consideration of the following illustrative examples.

EXAMPLE I In accordance with this example, there was provided 2,000 grams of copper powder having a particle size below 100 mesh. The copper powder was furnaced at a temperature of about l,400 F. to sinter it together to form a cake and to reduce the oxygen content. The sintered cake was broken up to produce a coarse, substantially oxygen-free, copper powder having a mesh size above 100 mesh. The material which had a particle size in the range of -50+80 mesh was separated from the remainder of the material by screening and comprised approximately 75 percent of the total. The 50+80 mesh material was moistened with polyethylene glycol and coated with tale. The coated masses were heated in dissociated ammonia at about 800 F. for minutes, thereby volatilizing the polyethylene glycol. The masses were then heated at l,200 F. for 15 minutes to form the uniform, dense, spherical, solid particles of the present invention. Approximately 75 percent of the particles obtained were in the desired 50+80 size range. The size distribution of the resulting particles is given in Table 1 below.

TABLE 1 Particle Size Yield, Percent EXAMPLE ll Example I was repeated with the exception that the desired ultimate product was in the size range of 40+60 mesh and the starting material was in this size range. The ultimate yield of product in the -40+60 range was approximately 70 percent. The product size distribution is shown in Table II below. The resulting material was spherical, fully dense and free of oxygen.

TABLE II Particle Size Yield, Percent EXAMPLE III In accordance with this example, the starting material was obtained by impinging air on a molten stream of copper. Starting material in the size range of 30+80 mesh was wetted with polyethylene glycol and coated with tale in a manner after Example l. The coated material was then heated in a manner after Example I to yield spherical, fully dense shot having no cracks and having substantially no oxygen therein. The resultant shot was brazed effectively. In contrast thereto, untreated material could not be brazed in view of the cracks appearing on the surface.

Over of the ultimate product was in the desired size range. The exact size distribution is shown in Table III below.

TABLE III Particle Size Yield, Percent The invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

l. A process for obtaining dense, spherical metal powders which comprises:

A. providing metallic particles having a particle size larger than 100 mesh,

B. covering said particles with a binder volatilizable above F. and at least 200 F. below the melting point of said metal particles,

C. coating said covered particles with a weld-preventing material, and

D. heating said coated particles in a protective atmosphere above the melting point of the particles to form a plurality of solid, dense, spherical, coherent metal spheres, and removing the weld-preventing material.

2. A process according to claim 1 wherein said particles are copper or copper base alloys.

3. A process according to claim 1 wherein the starting material metallic particles in step (A) are obtained by sintering a plurality of metal particles to form a cake and mechanically breaking up said cake.

4. A process according to claim 3 wherein the particles which are sintered have a particle size smaller than 100 mesh.

5. A process according to claim 1 wherein the starting material metallic particles in step (A) are obtained by disintegrating a stream of molten metal by impinging said stream with a fluid. 

2. A process according to claim 1 wherein said particles are copper or copper base alloys.
 3. A process according to claim 1 wherein the starting material metallic particles in step (A) are obtained by sintering a plurality of metal particles to form a cake and mechanically breaking up said cake.
 4. A process according to claim 3 wherein the particles which are sintered have a particle size smaller than 100 mesh.
 5. A process according to claim 1 wherein the starting material metallic particles in step (A) are obtained by disintegrating a stream of molten metal by impinging said stream with a fluid. 